1. Field of the Invention
This invention relates to a glue applicator for corrugated board manufacturing machines, and more particularly to a starch glue applicator in which a starch glue to be used for bonding a liner to a corrugated sheet is preheated to a predetermined temperature to shorten the time required for gelation adhesion of the starch glue, while realizing a significant reduction in equipment cost.
2. Description of the Prior Art
The corrugation machines intended for single-faced corrugated board or double-faced dual corrugated board generally employs a starch glue for a glue applicator which applies the glue on ridges of flutes of a corrugated core sheet to which a liner or facing sheet is to be bonded. Such starch glue is normally in the form of a suspension of low viscosity which is (in the case of the Stein Hall method) composed of a mixture of a main part containing starch and water in appropriate proportions and a carrier part containing starch and caustic soda in suitable proportions. The starch glue is stored in a glue pan which constitutes part of the glue applicator, and applied in a suitable amount on the ridge portions of a corrugated paper by means of an applicator roll. The corrugated sheet with the starch glue applied on the ridge portions of its flutes is bonded to a liner and then fed into a predetermined heating zone in which the applied glue is heated to a gelling temperature to develop its strong adhesive force. In this connection, the gelling temperature of a starch glue is about 60.degree. C. (though such varies depending upon its composition), but the starch in the glue pan is maintained approximately at ambient temperature so that it needs to be heated up to the gelling temperature by the use of a heater in order to develop its inherent adhesive force to guarantee a strong bond of the liner to the corrugated sheet. However, it is often the case that such a heater is extremely lengthy and has a drawback that it occupies a large space of a corrugator line.
For example, FIG. 1 schematically shows a glue applicator and a double backer mechanism for producing double-faced corrugated board. A pair of single-faced corrugated boards 10 and 12 produced respectively by single facers, which are located in upstream positions, are preheated through preheaters 14 and 16 on the way to glue applicators 18 and 20 where glue is applied to the ridges of the respective corrugated board. The back liner of the single-faced corrugated board 12 and a liner 24 which is fed through another preheater 22 are bonded to the ridges of the corrugations of the single-faced corrugated boards 10 and 12, respectively, between guide rolls 26 which are located downstream of the glue applicator. Each one of the glue applicators 18 and 20 is provided with an applicator roll 28 and a doctor roll 30 in the usual manner, applying the starch glue 34 in the glue pan 32 to the ridges of the single-faced corrugated board through the applicator roll 28. As mentioned hereinbefore, the starch glue 34 in the glue pan 32 is approximately at the ambient temperature, so that it has to be heated to the gelling temperature to produce its adhesive force. For this purpose, it has been the conventional practice to provide a heater over a large distance along the corrugator line, including heat boxes 36, a ballast roll 38 and a belt 40. The heating boxes 36 are constituted by a hollow box of iron casting with thick walls, and heated by internally flowing steam to transmit heat to glued portions of a double-faced corrugated board which is passed along the surfaces of the heat boxes, thereby attaining adhesion through gelation of the glue which is applied on the ridge portions of the corrugated paper. However, these days the corrugator lines are operated at high speeds, passing corrugated boards at a high speed through a heating zone which is constituted by the heat boxes 36. This naturally necessitates to provide a very lengthy heating zone in order to heat the corrugated board up to the gelling temperature of the starch glue. In other words, the provision of a lengthy heating zone has been unavoidably required to comply with the demand for the speed-up of operations. Thus, it has been a matter of great concern in the art to omit or minimize the heating zone which invariably occupies a large space in the conventional corrugation lines. Besides, the starch glue in the conventional glue applicators largely depends on the ambient temperature and therefore the corrugated board production efficiency is greatly influenced by variations in ambient temperature or by seasonal temperature variations. A difficulty is also encountered in that the corrugated sheets suffer from warping or other defects due to excessive heat transfer from the heat boxes 36 when the operational speed is slowed down.